Mobile · Intermediate
Offline-First Mobile App With Sync
Build a cross-platform notes-and-tasks app that works fully offline and syncs cleanly the moment a device reconnects.
You will build a cross-platform notes-and-tasks app using Expo and React Native that stores all data locally in SQLite so it works without any network connection. When the device reconnects, WatermelonDB or PowerSync handles conflict-aware sync to a Supabase Postgres backend automatically. This project teaches you the core patterns behind apps like Notion, Linear, and Superhuman that feel instant because they never wait for the network to respond to user actions.
What you build
- Create, edit, and delete notes and tasks entirely offline with no loading spinners on user actions
- Automatic background sync to Supabase when network connectivity is detected
- Conflict resolution that merges concurrent edits from multiple devices without data loss
- Real-time sync indicator showing pending changes count and last-synced timestamp
- Optimistic UI updates that reflect changes instantly before the server confirms them
- Multi-device support so the same account stays consistent across phone and tablet
What it teaches
- Local-first architecture and why writing to a local database before the network is the correct default for mobile apps
- WatermelonDB schema design, model classes, and reactive queries that re-render components when underlying data changes
- Sync protocol design including pull-push cycles, server-side timestamps, and soft deletes to handle conflict resolution
- Network state detection on mobile and how to trigger background work reliably using app lifecycle events
- Supabase Row Level Security policies to enforce per-user data isolation at the database layer
- EAS Build and how to produce signed development and production builds for physical device testing
How it works
- 1
User Action
- Write to local SQLite via WatermelonDB
- UI updates instantly (optimistic)
- 2
Local DB
- Change queued as pending sync record
- App stays fully usable offline
- 3
Network Check
- expo-network detects connectivity
- Sync triggered on reconnect or foreground
- 4
Sync Cycle
- Pull: fetch server changes since last sync
- Push: send local pending changes to Supabase
- 5
Supabase Postgres
- Merges changes, applies RLS
- Returns updated rows to all devices
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Take it further
- Add end-to-end encryption by encrypting note content with a per-user key before it leaves the device, so the server stores ciphertext only.
- Implement shared notebooks where multiple users can collaborate, requiring a CRDT or last-write-wins merge strategy for concurrent edits.
- Build a web companion using Supabase Realtime subscriptions so browser tabs update live while the mobile app syncs in the background.
